The invention relates to a color cathode ray tube system comprising
(a) an evacuated envelope having a neck, a cone and a display window,
(b) an electron gun in the neck, which gun has a beam-forming part for generating a central electron beam and two outer electron beams whose axes are co-planar, and a first and a second electrode system which in operation jointly constitute a main lens and are connectable to means for supplying an energizing voltage,
(c) an astigmatic element in the electron gun for astigmatically influencing the electron beams, and
(d) a deflection unit for generating line and field deflection fields for deflecting the electron beams.
Conventional color cathode ray tubes comprise self-convergent deflection units which in operation generate horizontal and vertical magnetic deflection fields in such a way that the three electron beams generated by the electron gun and focused on the display screen by the main lens converge throughout the display window. However, as a result, the electron beams are vertically overfocused on the display window. This can be partly compensated by means of a static astigmatic element in the gun. This is sometimes not sufficient for applications ;n which increasingly stricter requirements are imposed on the definition, for example, for high-resolution color picture display tubes. EP-A-0,231,964 describes a construction for an electron gun (a so-called DAF gun) which enables, by Dynamically coupling the strength of the Astigmatic element to the strength of the deflection fields a substantially complete correction of the vertical overfocusinq.
However, the horizontal spot growth also increases by a given spot enlargement factor in the case of deflection, which actor may be more than two in 110.degree. color cathode ray tube systems. The spot remains in focus or substantially in focus in the horizontal direction throughout the display screen. In the known construction this horizontal spot enlargement factor is not reduced or only reduced to a small extent. Due to the increasingly stricter requirements imposed on the definition of the image, notably in high-resolution color cathode ray tubes or when using color cathode ray tubes for high-definition television, it is also important to reduce the horizontal spot enlargement factor.